Hematopoietic stem cells comprise a very small fraction of bone marrow mononuclear cells, which retain the capacity for self-renewal as well as controlled differentiation into all the cellular elements of the peripheral blood. The molecular signals that confer the stem cell phenotype and the mechanisms that control stem cell proliferation and differentiation are incompletely understood. In preliminary work, we have identify a divergent homeobox gene (Pitx2) that is expressed in murine hematopoietic stem/progenitor cells but not in differentiated cells. Our studies have demonstrated the expression of a novel mRNA isoforms of this gene in hematopoietic stem/progenitor cells and embryonic stem cells. The research goals of this proposal are to establish the role of this gene in hematopoiesis. Specifically, we propose to: characterize the temporal and over-expression of the gene (products) on hematopoietic progenitor cell growth and differentiation in vitro as well as in vivo: study the effects of loss of function of the gene in ES cells carrying targeted disruptions of the gene as well as in various hematopoietic cells expressing a dominant negative version of Pitx2; and characterize the downstream gene targets of Pitx2 as well as the elements that regulate the expression of Pitx2.